Pipeline coupling

A pipeline coupling comprising a length of pipe having first and second ends thereto, each said end having an end face, a first substantially annular connecting piece at the first end of said length of pipe, an internally-threaded nut located on, to surround, said first connecting piece, a second substantially annular connecting piece at the second end of said length of pipe, an external screw-thread formed around said second end and corresponding with the internal thread of said nut, and a sealing ring for location between adjacent end faces of two lengths of pipe to be connected, the arrangement being such that, on securing of the nut onto the externally threaded portion of the second connecting piece, the two connecting pieces are drawn axially towards one another to compress the sealing ring therebetween and to effect a seal between the two connecting pieces, the coupling further comprising a substantially annular locking ring mounted on the second connecting piece to be movable axially therealong but means for rotatably fixing said annular locking ring relative thereto, corresponding projections and recesses being provided on said locking ring and on said nut on the first connecting piece and adapted to engage with one another to prevent relative rotation between the first and second connecting pieces, and releasable means for retaining the projections and recesses on the locking ring and on the nut in engagement with one another, said releasable means comprising a retaining ring having a rear surface thereto, a continuous circumferential groove being defined in the second connecting piece to receive said retaining ring, the retaining ring reacting between said groove and the rear surface of the locking ring and being selectively releasable from within said groove.